1. Field of the Invention
This invention relates to a miniaturized composite duplex filter for a radio communication equipment.
2. Description of the Prior Art
It is known in general to one skilled in the art that a dielectric filter is used for a duplexer of a radio communication equipment of a mobile phone and a portable telephone. Such a conventional dielectric filter has been designed to have a large cubic volume and occupy the greater part of the miniaturized radio communication equipment. The dielectric duplexer is basically composed of a band-pass filter at a receiving part and a band-pass filter or band-elimination filter at a transmission part.
FIG. 21 is one example of such a conventional dielectric duplexer which has been disclosed in U.S. Pat. No. 4,954,796. In FIG. 21, the dielectric duplexer is composed of two dielectric blocks, one being situated at the transmitting part 1 and the other being at the receiving part 8, and a transmission line connecting an output terminal of a transmitting dielectric filter and an input terminal of a receiving dielectric filter to an antenna terminal 7 is arranged to use striplines 9 and 10 on a mounting substrate, each length of which is set to have a high impedance in the mutual transmitting and receiving frequency.
As described above, the conventional duplexer has been made by a dielectric filter consisting of a plurality of dielectric resonators for the transmission and reception. For example, the transmitting duplex filter is provided with four steps of resonators, while the receiving duplex filter is provided with five steps of resonators respectively. As a radius of the dielectric resonator is in inverse proportion to an insertion loss of the filter, it was necessary for the radius of the dielectric resonator to be increased to make the insertion loss small and as a result, it has a disadvantage in that the shape must be increased accordingly. Finally, the reduction of the insertion loss means the increase of the shape of the dielectric filter and it is difficult to attain the miniaturization of said dielectric filter.